Technical Field
The present invention relates to an electronic thermostat, and more particularly to a heater used in an electronic thermostat which opens/closes a valve at a predetermined temperature to circulate coolant to maintain an automotive engine at a constant temperature, and a method for manufacturing an electronic thermostat using the same.
Description of the Related Art
Generally, an automotive thermostat is disposed between an engine and a radiator and automatically switches on/off in response to temperature change of a coolant to regulate coolant flow to the radiator, thereby maintaining the coolant temperature within a preset range.
In a mechanical thermostat, wax expands depending upon temperature of a coolant to operate a piston to open a valve.
Such a mechanical thermostat is operable in response to a switching on/off temperature set to a specified temperature. That is, the mechanical thermostat allows a valve to be opened/closed only at a preset temperature and thus cannot actively cope with demand for high performance/high efficiency vehicles and changes in driving environment or conditions.
To supplement shortcomings of such a mechanical thermostat, an electronic thermostat based on a variable control system capable of artificially controlling expansion of wax has been developed and applied to vehicles in practice.
An electronic thermostat actively controls temperature of an engine coolant according to a driving environment such as a load condition of a vehicle to maintain the coolant at an optimal temperature, and is superior to mechanical thermostats in terms of improvement in fuel efficiency and reduction in exhaust emissions.
The electronic thermostat employs a cartridge heater to actively control coolant temperature. Here, such a heater is mounted on a wax case and serves as a heat source artificially supplying heat to wax within the wax case, thereby increasing valve opening/closing speed.
In other words, the electronic thermostat further includes the cartridge heater which generates heat upon application of electric power, causing wax to expand, in addition to basic components of a typical mechanical thermostat, wherein the heating value of the cartridge heater is adjusted according to a driving environment of a vehicle such as driving speed, temperature of intake air, and engine load, thereby variably controlling valve opening/closing timing.
FIG. 1 is a sectional view of a typical heater for a thermostat mounted on a wax assembly and FIG. 2 is a perspective view of the heater for a thermostat of FIG. 1.
Referring to FIGS. 1 and 2, a typical cartridge heater for electronic thermostats 100 is inserted into wax W which expands and contracts in response to change in temperature within a wax case 9 contained in a thermostat to move a piston 20 back and forth, as shown in FIG. 1.
Here, the wax case 9 has a wax receiving space filled with the wax W and is formed at an upper portion thereof with a securing portion 15 on which the cartridge heater is mounted.
The cartridge heater 100 is mounted on the securing portion 15 of the wax case 9 by screw fastening using a fastener 150 and generates heat upon application of electric power, thereby selectively applying heat to the wax W.
Such a cartridge heater 100 includes a bobbin 110, a tube 120, insulation materials 130, an insulation cap 140, and the fastener 150.
First, lead wires 111 connected to an external power source are inserted into the bobbin 110 and a coil 113 connected to each of the lead wires 111 is wound around an outer circumferential surface of the bobbin 110.
Here, the lead wires 111 are connected to an external power source via a connector (not shown) such that the power source apply electric currents of different polarities to the lead wires, respectively, and the coil 113 is connected at one end thereof to one of the lead wire 111 and connected at the other end thereof to the other lead wire 111.
Accordingly, when supplied to the lead wires 111, electric current is delivered to the coil 113 via the lead wires 111 and flows through the coil 113, thereby allowing the coil 113 to generate heat.
The tube 120 is closed at one end thereof and open at the other end thereof. The bobbin 110 is mounted on the tube such that the other end of each of the lead wires 111 mounted on the bobbin 110 outwardly protrudes, with the bobbin 110 inserted into the tube through the open end of the tube towards the closed end of the tube.
The tube 120 is inserted into the wax W at the closed end thereof, increases temperature of the wax W using heat generated by the coil 113, and protects the coil 30 from chemical corrosion.
With the bobbin 110 inserted into the tube, the tube 120 is filled with the insulation materials 130. The insulation materials 130 are composed of magnesium oxide (MgO) powder.
Here, the magnesium oxide (MgO) powder maintains insulation between the tube 120 and the coil 113 to prevent current across the coil 113 from flowing into the tube 120 and prevents displacement of the coil while transferring heat generated by the coil 113 to the tube 120.
After the tube 120 is filled with the insulation materials 130 and the insulation cap 140 is mounted on the tube subsequent to insertion of the bobbin 110 having the lead wires 111 and the coil 113 mounted thereon into the tube 120, the tube 120 is shaped to have an outer diameter of a predetermined size and a curved surface at the closed end thereof by swaging.
The fastener 150 is mounted on the other end of the tube 120 to surround an outer circumferential surface of the other end of the tube 120 and is coupled to the wax case 9, thereby allowing the tube 120 to remain inserted into the wax 30.
Here, the tube 120 is press fitted into the fastener 150 with the closed end thereof protruding therefrom, whereby the tube 120 can be prevented from being separated from the fastener 150.
The fastener 150 is formed with threads N along an outer circumferential surface thereof and screwed to the securing portion 15 of the wax case 9 through the threads N.
In addition, a bush 160 securing the lead wires 111 protruding from the other end of the tube 120 may be mounted on the fastener 150.
However, such a typical heater for electronic thermostats, which uses the fastener 150 for coupling to the wax case and wax leakage prevention, has problems as follows.
A typical heater for thermostats structurally requires a fastener 150. This is because the fastener 150 functions as a coupling member through which the heater is coupled to a wax case and prevents wax leakage while serving as a negative terminal for grounding and is thus considered an indispensable component.
Accordingly, a process of manufacturing a thermostat requires manufacture and assembly of the fastener 150, and thus becomes complicated and requires many assembly operations, thereby causing deterioration in productivity while increasing manufacturing costs.
Specifically, manufacture of a thermostat using a typical heater for thermostats requires separate manufacture of a fastener to be coupled to a tube constituting the heater for thermostats, formation of a female thread on a wax case, and formation of a male thread corresponding to the female thread of the wax case on an outer surface of the fastener.
In addition, manufacture of a typical heater for thermostats must include inserting lead wires 111 into a bobbin 110, winding a coil 113 around the bobbin 110, and connecting the lead wires 111 to the coil 113, thereby providing a complex process.
Thus, use of the fastener for coupling the heater for thermostats to the wax case causes increase in the number of components and requires lots of complex manufacture and assembly processes, thereby causing deterioration in productivity while increasing manufacturing costs.
Further, when the typical thermostat is used for a long time, the thermostat often suffers from deterioration in clamping force due to loosening of a screw joint between the fastener and the wax case caused by vibration of a vehicle, thereby causing deterioration in wax leakage prevention.
Moreover, manufacture of the typical heater for thermostats requires lots of components and complex processes for manufacturing and assembling a heating unit, such as preparing a bobbin, inserting lead wires into the bobbin, winding a coil around the bobbin, and connecting the coil to the lead wires, which also causes deterioration in productivity and increase in manufacturing costs in manufacture of a thermostat.
Therefore, there has been continuous demand for an improved heater for thermostats that can increase productivity and reduce costs in manufacture of an electronic thermostat while enhancing durability and operational reliability when used in the thermostat.
Examples of the related art are disclosed in Korean Patent Publication No. 10-1497655 B and Korean Patent Publication No. 10-2013-0114505 A.